In recent years, it has become common to transmit large-volume data, such as still image data and moving image data in addition to audio data in cellular mobile communication systems, in response to spread of multimedia information. Active studies associated with techniques for achieving a high transmission rate in a high-frequency radio band has been conducted to achieve large-volume data transmission.
When a high frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher transmission rate can be expected within a short range. Accordingly, the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station”) decreases when a mobile communication system using a high frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band, while limiting an increase in the number of base stations.
In order to meet such a demand, studies have been carried out on a relay technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station”) is installed between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a mobile station not capable of directly communicating with a base station to communicate with the base station via a relay station.
An LTE-A (long-term evolution advanced) system for which the introduction of the relay technique described above has been studied is required to maintain compatibility with LTE (long term evolution) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well.
FIG. 1 illustrates example frames in which control signals and data are assigned in the LTE system and the LTE-A system.
In the LTE system, DL (downlink) control signals from a base station to a mobile station are transmitted through a DL control channel, such as PDCCH (physical downlink control channel). In LTE, DL grant indicating DL data assignment and UL (uplink) grant indicating UL data assignment are transmitted through PDCCH. DL grant reports that a resource in the subframe in which the DL grant is transmitted has been allocated to the mobile station. Meanwhile, in an FDD system, UL grant reports that a resource in the fourth subframe after the subframe in which the UL grant is transmitted has been allocated to the mobile station. In a TDD system, UL grant reports that the resource in a subframe transmitted after four or more subframes from the subframe in which the UL grant is transmitted has been allocated to the mobile station. In the TDD system, the subframe to be assigned to the mobile station, or the number of subframes before the assigned subframe in which the UL grant is transmitted is determined in accordance with the time-division pattern of the UL and DL (hereinafter referred to as “UL/DL configuration pattern”). Regardless of the UL/DL configuration pattern, the UL subframe is a subframe after at least four subframes from the subframe in which the UL grant is transmitted.
In the LTE-A system, relay stations, in addition to base stations, also transmit control signals to mobile stations in PDCCH regions in the top parts of subframes. With reference to a relay station, DL control signals have to be transmitted to a mobile station. Thus, the relay station switches the processing to reception processing after transmitting the control signals to the mobile station to prepare for receiving signals transmitted from the base station. The base station, however, transmits DL control signals to the relay station at the time the relay station transmits the DL control signals to the mobile station. The relay station therefore cannot receive the DL control signals transmitted from the base station. In order to avoid such inconvenience in the LTE-A, studies have been carried out on providing a region in which downlink control signals for relay stations are located (i.e., relay PDCCH (R-PDCCH) region) in a data region as illustrated in FIG. 2 in LTE-A. Similar to the PDCCH, locating DL grant and UL grant on the R-PDCCH is studied. In the R-PDCCH, as illustrated in FIG. 1, locating the DL grant in the first slot and the UL grant in the second slot is studied (refer to Non-patent Literature 1). Locating the DL grant in the first slot reduces a delay in decoding the DL grant, and allows relay stations to prepare for ACK/NACK transmission for DL data (transmitted in the fourth subframes following reception of DL grant in FDD).
Each relay station finds the downlink control signals intended for the relay station by performing blind-decoding on downlink control signals transmitted using an R-PDCCH region from a base station within a resource region indicated using higher layer signaling from the base station (i.e., search space).
As described above, the base station notifies the relay station of the search space corresponding to the R-PDCCH by higher layer signaling. Notification of the search space corresponding to the R-PDCCH may be performed in two different ways: (1) notification using a PRB (physical resource block) pair as a single unit; or (2) notification using an RBG (resource block group) as a single unit. The term, “PRB (physical resource block) pair” refers to a set of PRBs in the first and second slots, whereas the term, “PRB” refers to an individual PRB in either the first or second slot. Hereinafter, a PRB pair may simply be referred to as “PRB.” A resource block group (RBG) is a unit used when a plurality of PRBs are scheduled as a group. The size of an RBG is determined on the basis of the bandwidth of the communication system.
R-PDCCH has four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, refer to Non-patent Literature (hereinafter, abbreviated as “NFL”) 1). Levels 1, 2, 4, and 8 respectively have six, six, two, and two mapping candidate positions. The term “mapping candidate position” refers to a candidate region to which control signals are to be mapped. When a single terminal is set with one aggregation level, control signals are actually mapped to one of the multiple mapping candidate positions of the aggregation level. FIG. 2 illustrates example search spaces corresponding to R-PDCCH. The ovals represent search spaces at various aggregation levels. The multiple mapping candidate positions in the search spaces at the different aggregation levels are continuous on VRBs (virtual resource blocks). The mapping candidate positions in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.
In the LTE system, data resources used for transmission of DL or UL data are scheduled by dynamic scheduling or SPS (semi-persistent scheduling). In dynamic scheduling, a base station notifies a terminal of a data resource in each subframe with DL or UL grant. In SPS, upon notification of a data resource from a base station to a terminal with first control signals (DL or UL grant), a series of data is transmitted using predetermined resources in a group of transmission-scheduled subframes until the end of SPS is notified by second control signals. In SPS, two consecutive frames in the group of transmission-scheduled subframes have a predetermined frame interval. The predetermined resources are common between the transmission-scheduled subframes. In SPS, if dynamic scheduling of the data resources is indicated in any subframe in the group of transmission-scheduled subframes, a priority is given to the dynamic scheduling of the subframe, while data transmission of the data resource scheduled by SPS is skipped. SPS is suitable for communication involving small consecutive packets. An example communication involving small consecutive packets is speech communication. SPS applied to audio communication can eliminate the need for indication of the data resources used for mapping small packets using control signals every time, and thus can prevent an increase in the overhead of the control signals relative to the number of packets.